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A mysterious weather pattern that occurs over the Indian Ocean could allow weather forecasters to predict cyclones up to three weeks ahead of time.

Known as the Madden Julian Oscillation, forecasters have previously used the phenomenon to predict how active each cyclone season might be, and to forecast their behaviour about five days into the future once they form.

No crystal ball

But the oscillation is a diffuse, widespread weather pattern. Using it to forecast cyclones can dramatically improve forecasts, but it does not turn weather models into crystal balls.

Even armed with knowledge of the oscillation's influence on Hurricane Katrina, for instance, no one could have foreseen the storm's devastating strike on New Orleans.

"We may not be able to predict the strike of a storm at a given time and given location, but we can predict if the probability of a tropical storm strike will increase or decrease in the next few weeks over a large area," says Vitart.

"I think that the forecasting that would arise from the Madden Julian Oscillation would be different in nature from the three- and five-day operational track forecasts that currently come out of the National Hurricane Center, which predict the track (and cone of uncertainty) of a storm that already exists," says Dr Gabriel Vecchi of the National Oceanic and Atmospheric Administration.

"The Madden Julian Oscillation would help us to predict the genesis of a storm that doesn't exist yet, and the likely character of its track, landfall, etc."

Vitart says he is working on developing a way to use this type of forecasting to construct maps that display advanced warning of increased cyclone risk for a given stretch of islands or coastline.